Subtle differences between male and female Oystercatchers Haematopus ostralegus in feeding on the bivalve Macoma balthica

In this paper an analysis is made of subtle behavioural differences between adult male and female Ovstercatchers feeding on Macoma balthica under field conditions and in captivity. Macoma is a tellinid bivalve that in the Dutch Wadden Sea is mainly preyed upon during spring and summer when it is buried at a shallow depth. males lift Macoma more, whereas females handle them mostly in situ. Both sexes handle a Macoma in situ faster than one lifted. Time loss of males in handling more lifted Macoma is compensated by the larger size of lifted Macoma, which yields more flesh. The time the birds need to find an edible Macoma is similar for both sexes, resulting in equal mean food intake rates for males and females in the field. Lifted Macoma are generally hammered and, since males with their short strong bills are more likely to hammer bivalves than females, this difference in bill morphology might explain why males more often lift Macoma than do females, especially as hammering produces a blunt bill tip which would reduce efficiency at opening Macoma in situ. However, none of the selected bill morphology variables showed a relationship within the sexes that explained the differences between the sexes.